I. Field of the Invention
The present invention relates to wireless PBX and wireless local loop telephone systems. More specifically, the present invention relates to a novel and improved microcellular telephone system and distributed antenna system therefor so as to facilitate indoor communications using spread spectrum communication signals.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known in the art. However the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users each having a transceiver communicate through satellite repeaters or terrestrial base stations (also referred to as cell-sites stations, cell-sites or for short, cells) using code division multiple access (CDMA) spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
The terrestrial channel experiences signal fading that is characterized by Rayleigh fading. The Rayleigh fading characteristic in the terrestrial channel signal is caused by the signal being reflected from many different features of the physical environment. As a result, a signal arrives at a mobile unit receiver from many directions with different transmission delays. At the UHF frequency bands usually employed for mobile radio communications, including those of cellular mobile telephone systems, significant phase differences in signals traveling on different paths may occur. The possibility for destructive summation of the signals may result, with on occasion deep fades occurring.
Terrestrial channel fading is a very strong function of the physical position of the mobile unit. A small change in position of the mobile unit changes the physical delays of all the signal propagation paths, which further results in a different phase for each path. Thus, the motion of the mobile unit through the environment can result in a quite rapid fading process. For example, in the 850 MHz cellular radio frequency band, this fading can typically be as fast as one fade per second per mile per hour of vehicle speed. Fading this severe can be extremely disruptive to signals in the terrestrial channel resulting in poor communication quality. Additional transmitter power can be used to overcome the problem of fading. However, such power increases effect both the user, in excessive power consumption, and the system by increased interference.
The CDMA modulation techniques disclosed in U.S. Pat. No. 4,901,307 offer many advantages over narrow band modulation techniques used in communication systems employing satellite or terrestrial repeaters. The terrestrial channel poses special problems to any communication system particularly with respect to multipath signals. The use of CDMA techniques permit the special problems of the terrestrial channel to be overcome by mitigating the adverse effect of multipath, e.g. fading, while also exploiting the advantages thereof.
In a CDMA cellular telephone system, the same wide band frequency channel can be used for communication in all cells. The CDMA waveform properties that provide processing gain are also used to discriminate between signals that occupy the same frequency band. Furthermore the high speed pseudonoise (PN) modulation allows many different propagation paths to be separated, provided the difference in path delays exceed the PN chip duration, i.e. 1/bandwidth. If a PN chip rate of approximately 1 MHz is employed in a CDMA system, the full spread spectrum processing gain, equal to the ratio of the spread bandwidth to system data rate, can be employed to discriminate against paths that differ by more than one microsecond in path delay from each other. A one microsecond path delay differential corresponds to differential path distance of approximately 1,000 feet. The urban environment typically provides differential path delays in excess of one microsecond, and up to 10-20 microseconds are reported in some areas.
In narrow band modulation systems such as the analog FM modulation employed by conventional telephone systems, the existence of multiple paths results in severe multipath fading. With wide band CDMA modulation, however, the different paths may be discriminated against in the demodulation process. This discrimination greatly reduces the severity of multipath fading. Multipath fading is not totally eliminated in using CDMA discrimination techniques because there will occasionally exist paths with delayed differentials of less than the PN chip duration for the particular system. Signals having path delays on this order cannot be discriminated against in the demodulator, resulting in some degree of fading.
It is therefore desirable in the CDMA cellular telephone system that some form of diversity be provided which would permit a system to reduce fading. Diversity is one approach for mitigating the deleterious effects of fading. Three major types of diversity exist: time diversity, frequency diversity and space diversity.
Time diversity can best be obtained by the use of repetition, time interleaving, and error detection and correction coding which is a form of repetition. The present invention employes each of these techniques as a form of time diversity.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in copending U.S. patent application entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", Ser. No. 07/433,030, filed Nov. 7, 1989, now U.S. Pat. No. 5,101,501 issued Mar. 31, 1992 and copending U.S. patent application entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", Ser. No. 07/432,552, also filed Nov. 7, 1989, now U.S. Pat. No. 5,109,390 issued Apr. 28, 1992, both assigned to the assignee of the present invention.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in copending U.S. patent application entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", Ser. No. 07/433,031, filed Nov. 7, 1989, now U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, also assigned to the assignee of the present invention.
The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 contemplated the use of coherent modulation and demodulation for both directions of the link in mobile-satellite communications. Accordingly, disclosed therein is the use of a pilot carrier signal as a coherent phase reference for the satellite-to-mobile link and the cell-to-mobile link. In the terrestrial cellular environment, however, the severity of multipath fading, with the resulting phase disruption of the channel, precludes usage of coherent demodulation technique for the mobile-to-cell link. The present invention provides a means for overcoming the adverse effects of multipath in the mobile-in-cell link by using noncoherent modulation and demodulation techniques.
The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 further contemplated the use of relatively long PN sequences with each user channel being assigned a different PN sequence. The cross-correlation between different PN sequences and the autocorrelation of a PN sequence for all time shifts other than zero both have a zero average value which allows the different user signals to be discriminated upon reception.
However, such PN signals are not orthogonal. Although the cross-correlations average to zero, for a short time interval such as an information bit time the cross-correlation follows a binomial distribution. As such, the signals interfere with each other much the same as if they were wide bandwidth Gaussian noise at the same power spectral density. Thus the other user signals, or mutual interference noise, ultimately limits the achievable capacity.
The existence of multipath can provide path diversity to a wideband PN CDMA system. If two or more paths are available with greater than one microsecond differential path delay, two or more PN receivers can be employed to separately receive these signals. Since these signals will typically exhibit independence in multipath fading, i.e., they usually do not fade together, the outputs of the two receivers can be diversity combined. Therefore a loss in performance only occurs when both receivers experience fades at the same time. Hence, one aspect of the present invention is the provision of two or more PN receivers in combination with a diversity combiner. In order to exploit the existence of multipath signals, to overcome fading, it is necessary to utilize a waveform that permits path diversity combining operations to be performed.
A method and system for constructing PN sequences that provide orthogonality between the users so that mutual interference will be reduced is disclosed in copending U.S. patent application entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", Ser. No. 07/543,496, filed Jun. 25, 1990, now U.S. Pat. No. 5,103,459 issued Apr. 7, 1992 also assigned to the assignee of the present invention. Using these techniques in reducing mutual interference allowing higher system user capacity and better link performance. With orthogonal PN codes, the cross-correlation is zero over a predetermined time interval, resulting in no interference between the orthogonal codes, provided only that the code time frames are time aligned with each other.
In an such a CDMA cellular mobile system as disclosed in Ser. No. 07/543,496, signals are communicated between a cell-site and mobile units using direct sequence spread spectrum communication signals. In the cell-to-mobile link, pilot, sync, paging and voice channels are defined. Information communicated on the cell-to-mobile link channels are, in general, encoded, interleaved, bi-phase shift key (BPSK) modulated with orthogonal covering of each BPSK symbol along with quadrature phase shift key (QPSK) spreading of the covered symbols. In the mobile-to-cell link, access and voice channels are defined. Information communicated on the mobile-to-cell link channels are, in general, encoded, interleaved, orthogonal signalling along with QPSK spreading. Using orthogonal PN sequences does in fact reduce mutual interference, thereby permitting greater user capacity, in addition to supporting path diversity so as to overcome fading.
The above mentioned patent and patent applications disclose a novel multiple access technique wherein a large number of mobile telephone system users communicate through satellite repeaters or terrestrial base stations using code division multiple access spread spectrum modulation that allows the spectrum to be reused multiple times. The resulting system design has a much higher spectral efficiency than can be achieved using previous multiple access techniques.
In cellular telephone systems, a large geographic area is provided with mobile telephone service by installing a number of cell-sites situated so as to provide coverage of the entire geographic area. If service demand exceeds the capacity that can be provided by a set of cell-sites that just provides coverage, the cells are subdivided into smaller cells. This process has been carried out to the extent that some major metropolitan areas have nearly 200 cell-sites.
The technique described in U.S. Pat. No. 4.901,307 uses CDMA to achieve a very high capacity by providing marginal isolation gain through the exploitation of system characteristics and functions such as multiple steerable antennas, speech activity and reuse of the entire frequency band in every cell of the system. The result is a significantly higher system capacity than provided by other multiple access techniques such as FDMA and TDMA.
In a further development of the cellular telephone idea, it is desired to provide a number of very small cells, called microcells, which would provide coverage of a very limited geographic area. Usually, it is considered that such areas are limited to a single floor of an office building and the mobile telephone service can be viewed as a cordless telephone system that may or may not be compatible with the mobile cellular telephone system. The rationale for providing such a service is similar to the reasoning for use of Private Branch Exchange (PBX) systems in business offices. Such systems provide for low cost phone service for a large number of calls between phones within the business while providing simplified dialing for internal phone numbers. A few lines are also provided to connect the PBX system to the public telephone system, allowing calls to be made and received between phones in the PBX system and telephones located elsewhere. It is desirable for the microcell system to provide a similar level of service but with the added feature of cordless operation anywhere within the service area of the PBX.
In applications such as the wireless PBX or Wireless local loop telephone systems path delays are much shorter in duration than in cellular mobile systems. In buildings and other indoor environments where PBX systems are used it is necessary to provide a form of diversity which will enable discrimination between CDMA signals.
The primary problem solved by the disclosed invention is the provision of a simple antenna system that provides high capacity, simple installation, good coverage and excellent performance. Another problem is to achieve the above limited coverage while maintaining compatibility with the mobile cellular system and while taking a negligible amount of capacity away from the mobile system. This is achieved in the disclosed invention by combining the capacity properties of CDMA with a new distributed antenna design that confines the radiation to a very limited and carefully controlled area.
The implementation of spread spectrum communication techniques, particularly CDMA techniques, in a PBX environment therefore provides features which vastly enhance system reliability and capacity over other communication system techniques. CDMA techniques as previously mentioned further enable problems such as fading and interference to be readily overcome. Accordingly, CDMA techniques further promote greater frequency reuse, thus enabling a substantial increase in the number of system users.